This review on isomers or acyl glucuronides (iso-glucuronides) updates earlier reviews, and attempts to place in context the advances that have been made, especially over the last 15 years. The essential chemistry behind the intramolecular acyl migration and anomerization reactions of acyl glucuronides has been appreciated for 30 years. The great advances in the past 15 years have been in understanding the dynamics and kinetics of these processes in vitro, using highly sophisticated modern technology, e.g. LC-NMR, LCMS/ MS. In this way, earlier assumptions on kinetics and identification of migration isomers and anomers have come under intense review and update. Extensive structure-activity relationships, involving electronic and steric characteristics of an acyl glucuronide and its possible 7 isomers (excluding transient open-chain species) have been delineated. The covalent modification of endogenous proteins and other macromolecules has been further explored, though direct linkage between such modification and toxic sequelae remains elusive. An alternative view of acyl glucuronides and iso-glucuronides as just xenobiotics has perhaps added the dimension that acyl glucuronidation (and attendant formation of iso-glucuronides) does not necessarily mean that glucuronidation of the aglycone has ended metabolic sequences in vivo.